A typical feature in members of the mustelid family is sexual size dimorphism, related to the different reproductive strategies of the sexes. As a consequence sexual divergence in many ecological and behavioural traits is expected, including temporal strategies. In line with this prediction, sex differences in the diel activity pattern of the European polecat Mustela putorius, a dimorphic mustelid, were quantified and tested. Twenty-eight polecats were captured in a rural area of central Italy (Abruzzo region) and 10 males and six females were monitored in the field using radio-telemetry techniques for 81–621 days each. Males and females showed a pronounced dimorphism in size (males/females mass ratio=1.6) within the known range for the species. Multivariate analysis of diel time preference shows that temporal behaviour differed significantly and markedly between the sexes. Males exhibited a regular and constant nocturnal pattern in every season, preferring the time ranging from 20:00 to 06:00; activity was lower at dawn and dusk, and scarce during daylight hours. Females were significantly diurnal and crepuscular but individuals revealed flexibility in activity pattern, including polyphasic or arrhythmic patterns without apparent temporal organization. The constraints of small body size and reproduction on female activity, and the sex segregation in activity timing are discussed in the context of intrasexual territoriality.